Brain tumors are growths of abnormal cells in the tissues of the brain and can be benign (noncancerous) or malignant (cancerous). The estimated number of new cases of brain and other nervous system tumors in the United States in 2008 is 21,810, and an estimated 13,070 related deaths are expected (NCI, 2008).
Gliomas, which are primary brain tumors, form in the glial cells of the brain or spinal cord and can spread throughout the nervous system. They can be benign or malignant. There are several types of gliomas, such as astrocytomas, ependymomas, and oligodendrogliomas. The committee examined several studies of brain tumors and TBI, and they are discussed in this section.
The committee reviewed 14 primary studies that examined the association between TBI and brain tumors. Most relied on self-reports of TBI to assess exposure. Three of the 14 studies—in Olmsted County, Minnesota (Annegers et al., 1979), Denmark (Inskip et al., 1998), and Sweden (Nygren et al., 2001)—used medical records to ascertain TBI and therefore are substantially less prone to recall bias.
In a retrospective cohort study, Annegers et al. (1979) followed 2,953 patients in Olmsted County, Minnesota, who had a diagnosis of TBI during 1935–1974 to determine the occurrence of later brain tumors. Patients were selected from among 3,587 head-injured people in the population of Olmsted County who had survived the initial injury and had no history of pre-existing brain tumor. TBI was defined as brain involvement manifested by loss of consciousness (LOC), amnesia, or skull facture. Four brain tumors were observed in those with TBI, compared with an expected 4.13 (RR, 1.0; 95% CI, 0.3–2.6). There were one astrocytoma (SMR, 0.7) and three meningiomas (RR, 1.6; 95% CI, 0.3–4.7). The tumors were not associated with TBI severity and were diagnosed 5–16 years after the injury.
In a study of the incidence of brain tumors after TBI in Denmark (Inskip et al., 1998), nationwide registry of hospital discharges in 1977–1992 was used to identify 228,005 people who had been hospitalized for TBI on the basis of ICD-8 codes for fractured skull, concussion, or cerebral laceration or contusion. This registry was linked with the Danish Death Certificate File and the Danish Cancer Registry for 1977–1993 to determine the incidence of cancer. The incidence was compared with that in the Danish population to obtain standardized incidence ratios (SIRs), which were adjusted for sex, age, and calendar year. Concussion and fractured skull were the most common injuries and typically resulted from traffic accidents, falls, and sports-related activities. There were 299 tumors of the brain and nervous system in Denmark during the 7-year period, of which 261 were intracranial tumors, including 113 gliomas, 36 meningiomas, 12 neurilemmomas, 8 medulloblastomas, and 16 vascular tumors. The SIR for any intracranial tumor associated with TBI was 1.36 (95% CI, 1.20–1.53), and most tumors occurred in the first year after the injury (SIR, 3.38; 95% CI, 2.59–4.34), regardless of tumor type. The SIR dropped closer to 1.0 in later years (SIR, 1.15; 95% CI, 0.99–1.32). Sixty-two intracranial tumors were diagnosed in the first year of followup; 43 of these were diagnosed during the first 6